Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as a client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station with which a client terminal is communicating is referred to as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred to as a neighbor cell.
Cellular wireless communication systems may use multiple frequencies within a cell and across different cells. If a neighbor cell is using the same frequency as the serving cell, it is referred to as an intra-frequency neighbor cell. If a neighbor cell is using a frequency different from the serving cell, it is referred to as an inter-frequency neighbor cell. A network may use different Radio Access Technologies (RATs) for providing various services. In a particular network, the cells of different RAT's may be overlapping or adjacent to each other. If a neighbor cell is using a RAT that is different from the RAT used by the serving cell, it is referred to as an inter-RAT neighbor cell.
When a client terminal is not in active call or active data transfer, it is considered to be in idle mode. In idle mode, when a client terminal moves away from the coverage area of its current serving cell and it may be in the coverage area of one or more neighbor cells, it may initiate a cell reselection procedure towards one of the neighbor cells. The neighbor cell towards which the cell reselection procedure is initiated is referred to as a target cell. After the cell reselection procedure is completed successfully, the target cell becomes the new serving cell for the client terminal.
In idle mode, a client terminal for the most part may turn off a majority of its circuitry to reduce power consumption. This is often referred to as “sleep” state, a sub-state within the idle mode. A client terminal may remain in a sleep state for a long duration and may wake up at the desired time window where it expects to receive the paging messages and certain broadcast messages. The success rate for the incoming calls (e.g., mobile terminated voice calls and/or data calls) in a client terminal is directly related to the successful reception of paging messages. Normally, in idle mode, in addition to the reception of paging messages and broadcast messages, a client terminal may continue to search and monitor neighbor cells.
When a client terminal performs a network registration procedure to get service from a cell, it decodes the system information broadcast in the cell. The client terminal may store such system information for most recently visited cells for future reference. The system information carries important information regarding the cell reselection criteria. Such information includes but not limited to the tracking area code (TAC), Public Land Mobile Network Identity (PLMN ID), Closed Subscriber Group Identity (CSG ID), etc. This information may help to decide whether a cell is a “suitable cell” for the client terminal to avail all the services offered by it, or the cell is an “acceptable cell” where the client terminal may avail only limited services such as emergency calls. The client terminal may have to perform the network registration procedure to get service from the network and for the network to be able to page the client terminal for mobile terminated (incoming) calls. The network registration procedure may be typically performed by a client terminal with a particular cell. A group of cells in the geographic vicinity of each other may form a registration area. A registration area may be identified based on the system information of a cell. For example, the TACs of a group of cells in a registration area may be the same.
As long as a client terminal is in the same registration area, the client terminal may not perform network registration procedure again except that a periodic network registration update procedure may need to be performed even if the client terminal continues to move within the same registration area. If the client terminal moves to a cell which does not belong to the registration area in which the client terminal is registered, then the client terminal may perform network registration procedure to get access to and service from the network.
When a client terminal is in idle mode, the network may only be aware of the location of the client terminal at the registration area level. In order for a network to page a client terminal, it may need to send the paging message in all cells belonging to the same registration area. The instances for paging message transmission, known as paging occasions (POs), are usually derived based on a client terminal's unique identity and a paging cycle. The network may transmit a paging message addressed to a specific client terminal in its specific paging occasion. When a client terminal performs cell reselection, the exact instances of paging occasions may be different in the new serving cell. Depending on the exact timing of the cell reselection and the timing of the POs in the old serving cell and the new serving cell, the client terminal may potentially miss a paging message or may receive it with delay.